Relevant literature, including the references noted parenthetically in the following discussion, are listed in an Appendix to this specification.
Trends in the manufacture of integrated circuits (IC), flat panel displays (FPD), and other related industries are driving the need for further advances in cleaning methods. Cleaning of product substrates is a critical part of the manufacturing process. Contamination remaining on the substrate may cause defects rendering the end product device useless. Contaminant particles can cause killer defects and lower production yield. And, with further advances in design and manufacturing processes, it is expected that sub-micron particles will have an even greater impact on production yield than they do today.
Present liquid-phase and vapor-phase cleaning methods are not entirely satisfactory in removing sub-micron particulates. Indeed, such methods are themselves a potential source of such contamination.
"Dry cleaning" techniques, i.e. cleaning without liquids or vapors, are called for by environmental concerns and by advances in manufacturing processes of all kinds. Dry cleaning technologies presently use plasmas and some vapor/gas phase techniques for vacuum process applications.
Silicon wafers have for some time been cleaned by high energy pulsed ruby lasers (emitting at 694.3 nm) and Nd:YAG lasers (1.06 micron). These relatively long wavelengths are not well absorbed by the silicon substrates, so higher laser power is required. This in turn can lead to undesirable effects such as substrate and particle melting. Also, coherence of the laser light creates interference effects and non-uniform beam profiles, with resulting hot spots and substrate surface damage.
Adhesion forces exist between particles and surfaces due to Van der Waals, electrostatic, and capillary attraction mechanisms. Van der Waals forces are due to interaction between an intrinsic dipole moment in one body and an induced dipole in a nearby body. Electrostatic forces are due to charge transfer and the subsequent formation of an electrostatic double layer repulsion between particle and substrate. Capillary forces arise when atmospheric moisture condenses in the gap between particle and substrate, and are a function of particle radius and liquid surface tension.
The mass (m) of a particle varies with the cube of its diameter, and the force (F) of its adhesion to a surface varies directly with its diameter. Hence, the acceleration (a=F/m) required to detach a particle from a surface varies inversely with the square of the diameter. As a result, adhesion forces on a micron-scale particle greatly exceed gravitational forces. One investigator (Tam) reports that the Van der Waals force on a 1 micron particle exceeds the weight of the particle by a factor of 10.sup.7. This greatly exceeds the forces characteristic of conventional cleaning methods. Another investigator (Bowling 1985) notes that the contact area of a particle on a surface is very small, and so the pressure on that particle-surface contact area is correspondingly very high. For a typical 1 micron particle, Bowling estimates a force per unit area of approximately 10.sup.9 dynes/cm.sup.2, which is enough pressure to deform the particle. To the extent of such deformation, the contact area is increased, and the particle-to-surface adhesion strengthened.
Many substances absorb radiation strongly in the UV range, in short absorption path lengths. The development of UV lasers made it possible to irradiate a substrate so that only a thin surface layer is heated. This serves to concentrate the energy of the process where it is needed, at the particle-substrate interface region. This, in turn, relaxes laser energy requirements, and helps to minimize substrate surface damage.
Laser methods of particle desorption may be classified as being ablative or non-ablative. Ablation is an energetic phase transformation from solid to gas. At high enough laser intensities, a thin surface layer of substrate is removed, carrying surface contaminants away with it. As a cleaning method, this process is somewhat analogous to chemical etching because a thin surface layer of material is removed. Like chemical etching, laser ablation tends to micro-roughen the surface, making the process generally unsuitable for semiconductor or FPD cleaning applications.
Non-ablative particle removal methods rely on particle-substrate interactions excited by light to break the physical or chemical bonds holding the particle to the substrate. Non-ablative methods may be distinguished by whether or not an energy transfer medium is used to mediate the particle-substrate interaction. That is, they are subclassified as either direct exposure methods (no energy transfer medium) or energy transfer methods (with an energy transfer medium).
In a direct exposure method, a UV laser directly irradiates both particulate and surrounding substrate area (Magee 1991). Though it is not entirely understood, the method acts photochemically to break surface bonds (Engelsberg 1993 a,b,c) and to induce rapid thermal expansion of the particulate and/or a thin substrate layer to forcefully eject the particle from the surface (Kelley, Magee 1991).
In an energy transfer method, the substrate is coated with a layer of liquid energy transfer medium (Allen, Lee, Zapka). Radiation of the coated substrate causes the liquid layer to vaporize explosively, taking surface particulate with it.
Another recent development in laser cleaning, the "Radiance Process".TM., is a surface cleaning process using deep ultraviolet (DUV) radiation and an inert gas flowing over the surface. Its theory of operation is that the application of DUV at a prescribed energy density initiates a localized photon/phonon interaction under the beam area. Applying photon radiation to the surface creates the cleaning phenomenon as long as the photon energy and density remain below the ablation threshold of the material. The Radiance Process is further disclosed in U.S. Pat. Nos. 5,024,968 and 5,099,557 to Engelsberg.